1. Field of the Invention
This invention relates generally to the field of external counterpulsation.
2. Background of the Invention
Cardiac disease remains a significant health problem in the United States and in the world. Although there are a variety of pharmacological and interventional therapies to treat cardiac disease, many patients are not adequately helped by traditional treatments. In particular, the impaired healths of many cardiac disease patients create a substantial risk of morbidity and mortality for interventional therapies such as coronary bypass surgery. Unsuitable coronary anatomy, prior revascularization attempts or other comorbid conditions may still preclude less-invasive therapies such as percutaneous transluminal coronary angioplasty. Thus, the development of non-invasive therapies may provide additional health benefits to patient populations that cannot tolerate or have gained limited benefits from traditional treatments.
External counterpulsation (ECP) is a technique that has demonstrated effectiveness in treating angina and congestive heart failure (CHF). ECP is an outgrowth of research from the 1950's directed at augmenting the low cardiac output of patients with advanced cardiac disease. External counterpulsation is a noninvasive procedure whereby cuffs are placed around the lower extremities of the body, inflated during the filling phase of the heart, and rapidly deflated during the contractile phase. During the filling or diastolic phase of the heart, the chambers of the heart are passively filled with venous blood before the next contraction. By rapidly inflating the cuffs during diastole, venous pressure is increased increased in the peripheral regions of the body and venous blood return to the heart is enhanced. This increased ventricular filling or preloading results in an increased ejection of blood from the ventricles during the next systolic phase, which can enhance the cardiac output. Increased arterial pressure during diastole may also enhance filling of the coronary arteries. The rapid deflation of the cuffs during the period of systole or contraction lowers the peripheral vascular resistance (PVR) which the heart pumps against and further enhances cardiac output. A reduction in PVR lessens the workload of an impaired heart by decreasing the effort used to maintain the forward flow of blood. To further enhance limb compression, portions of the limbs may be compressed sequentially from the distal limbs to the proximal limbs, rather than all portions simultaneously, to increase venous return of blood to the heart. The synchronization of inflation and deflation with the resting and contractile phases of the heart has been shown to increase blood flow to many vascular beds, including the coronary arteries. Furthermore, by increasing the diastolic pressure component of the mean perfusion pressure of the body tissues, the systolic pressure component used to maintain mean perfusion pressure may be reduced to further lower the workload of the heart. When external counterpulsation is performed, plethysmographic tracings of the blood pressure waveform will show a decrease in the systolic peak and an increase in the diastolic peak. A diastolic-to-systolic effectiveness ratio, calculated by dividing the peak diastolic amplitude by the peak systolic amplitude, is commonly used to measure the hemodynamic changes induced by external counterpulsation.
Interestingly, although the standard ECP treatment consists of thirty-five hours of treatment over seven weeks, the benefits of ECP persist beyond the thirty-five hours during which ECP is applied to a patient and may benefit more than just the cardiovascular system. It has been hypothesized that the limited duration of enhanced blood flow may increase the shear stress in the endothelial walls of the vasculature. Shear stress is considered a major stimulus for angiogenesis and may upregulate the production of growth factors such as Vascular Endothelial Growth Factor and Hepatocyte Growth Factor. This shear stress also increases endothelial release of nitric oxide, which may have vasodilatory, anti-platelet, anti-thrombotic, anti-proliferative and anti-inflammatory effects on the vasculature. Research also suggests that nitric oxide may have beneficial antioxidant effects.